The present invention relates to an electric conducting cable which is insensitive to nuclear radiation. This invention more particularly applies in the case of nuclear reactors to the continuous measurement of different physical magnitudes, such as the flux of thermal or fast neutrons, the gamma radiation intensity, deformations, etc. Generally, these measurements are performed by means of probes which supply an electrical signal which must be transferred from the probe, which is sometimes submerged in the reactor core, to a measuring apparatus located in an external control room. The conducting cables used are often disturbed by strap or leakage currents or voltages and in particular in that part of the said cables located within the reactor core or very close to the latter under the influence of radiation emitted by the fuel, namely neutrons and deactivation .beta. rays, gamma rays and electrons emitted at the time of gamma radiation capture. These disturbances can be considerable, particularly if the irradiated length of the cable is relatively great or if the probe has a low sensitivity compared with that of the cable. Due to these disturbances it is virtually impossible to carry out certain measurements such as, for example, the continuous measurement of the fast neutron flux. The interfering signals resulting from these disturbances also make the detector unusable for a certain time when it is a question of performing measurements of fast transient phenomena which must be known in order to improve the safety of fast reactors. These stray or leakage currents and voltages often make it necessary to use detectors with a very large volume if it is desired to instantaneously measure the thermal neutron flux.
Various solutions have been envisaged for eliminating this excessive sensitivity of cables which transmit signals between measuring probes and measuring apparatus.
According to one of these solutions for obtaining a useful signal well above the noise produced in the cable involves increasing the detector sensitivity by increasing its dimensions. However, this solution cannot always be envisaged, particularly when the space necessary for housing this large detector is not available, which also disturbs the radiation field to be measured. Moreover, this solution which does not attack the cause of the interference, does not make it possible to develop detectors having a low relative sensitivity, for example for measuring fast neutron fluxes.
Another solution consists of using a bifilar shielded conducting cable. The two internal conductors are assumed to carry the same interfering current, one of the two conductors being connected to the detector, whilst the other which is the compensating conductor, is not connected thereto. By subtracting the signal of the compensation conductor from the signal produced by the detector and its connecting conductor, it is possible to become partly independent of the interfering signal produced in the cable. However, this solution is not perfect. Thus, it is more difficult and costly to measure two signals and to obtain the difference between them and to carry out a single measurement. Moreover, it is known that a magnitude obtained by subtracting two similar magnitudes can easily suffer from a very large relative error. Finally, it is very difficult and even impossible to manufacture a symmetrical, bifilar cable, so that the two conductors never supply the same interfering signal and uses are more likely to use two monofilar shielded cables rather than a single bifilar cable.
In the solution using two monofilar shielded cables, one is used for measurement and the other for compensation. This solution has the main disadvantage of doubling the number of measuring wires, which leads to the doubling of the overall dimensions, the number of sealed packages, etc, thus increasing the total cost of such measurements. This solution also requires the two cables to follow the same path in such a way that they are located in the same radiation fields. However, this constraint is not always respected, so that different interfering signals are produced in each of the monofilar cables. Finally, as in the case of the bifilar cable, it is necessary to perform the measurements by forming the difference, which leads to a very high relative error.